I would like to know the threading of the hex plug in a Fiat 8v TC cylinder head. I want to remove the plug and install a temperature sensor for my Megajolt system. Also I am having some difficulties removing the plug. It's probably been in there since 1977 and the walls of the plug are damaged. Are there any issues with applying some heat in this area?
dlh98370

You may find it is only possible to remove it by drilling out and rethreading, that is my exp anyway. There is no problem wiht heating it up - I would use oxy-acetylene - in a very localised way. Brief application of very intense heat just around the plug (bringing the alloy almost to melting point) would be the only way heat would help. You would have to have the socket in place ready to apply force to the plug and the head clamped to stop it rotating..

If it's the same as the water outlet in the top of the head then strangely it's a BSP (British Std pipe??) IIRC. The water outlet on a head I was doing was damaged so I had to get it out, very carefully. It's now been replaced with some plumbing fittings from B&Q, worked a treat with a bit of thread tape and been leak free ever since.

I had the same problem on my head. I was able to remove the plug by drilling, without damage the thread. I drilled larger and larger until loosening of the plug. The plug thread is M16x1.5, slightly conical plug.

I'm used M16x1,5 tapered temp sender in TC head. Fits nicely, similar like in head which has 100% certain M16x1.5 taper threads.
But if I look both M16x1,5 and 3/8 BSPT threads, they are pretty close to each other.

I had some gauge adaptors made some years back - the guy who cnc machined them was very skilled and had a huge range of thread gauges to aid him. He went thru several before declaring he was going with 3/8 BSPT.

I noticed according to photos from Your book and from this forum that You always remove a hex plug from TC head. Could You tell me a reason why You are doing it ? Removing it is a bit tricky, the only reason for removing it is in my opinion for a better coolant passages cleaning.
When You fit a new plug (stainless ?) which Loctite You are using, a 542 ? Do You remove also a hex plugs from a front of the head ?